This invention relates to a fuel cell system which uses the waste heat from hot exhaust coming out of fuel cell stacks to increase the overall fuel cell efficiency. More particularly, the present invention focuses on a method and apparatus to simplify the heat exchange to recuperate the high temperature gas at the outlet of a fuel cell without the complexity of an external recuperator or heat exchanger. The current invention may also eliminate a recuperator from a fuel cell system while maintaining the heat recuperation efficiency to increase the system reliability and maintainability. In effect, the current invention replaces complicated and costly heat exchanger or multiple heat exchangers and associated complex piping and ducting.
Fuel cells electrochemically react fuels with oxidants to generate electricity. The key components in a fuel cell include a cathode material, an electrolyte material, and an anode material. The electrolyte is a non-porous material sandwiched between the cathode and anode materials. The fuel and oxidant fluids are typically gases and are continuously passed through separate passageways. A fuel gas can be hydrogen, a short chain hydrocarbon, or a gas containing desired chemical species in some form. An oxidant may be an oxygen containing gas, or quite commonly air. Reactant gases, fuels and oxidants, are typically pre-heated before being fed to electrolyte. Electrochemical conversion occurs at or near the three-phase boundaries of each electrode (cathode and anode) and the electrolyte. The fuel is electrochemically reacted with the oxidant to produce a DC electrical output. The anode or fuel electrode enhances the rate at which electrochemical reactions occur on the fuel side. The cathode or oxidant electrode functions similarly on the oxidant side.
One of the common constructions of fuel cells is a solid oxide fuel cell (SOFC) that uses solid electrolytes for power generation. Solid electrolytes are either ion conducting ceramic or polymer membranes. In the former instance, the electrolyte is typically made of a ceramic, such as dense yttria-stabilized zirconia (YSZ) ceramic, that is a nonconductor of electrons, which ensures that the electrons must pass through the external circuit to do useful work. With such an electrolyte, the anode is oftentimes made of nickel/YSZ cermet and the cathode is oftentimes made of doped lanthanum manganite.
SOFCs of various construction geometries have been designed. These include the tubular, segmented cells in series, and planar geometries. These various constructions are described in xe2x80x9cCeramic Fuel Cellsxe2x80x9d by N. Q. Minh, Journal of the American Ceramic Society, 76, p. 563, 1993.
Sometimes, a planar construction resembles a cross-flow heat exchanger in a cubic configuration. The planar cross flow fuel cell is built from alternating flat single cell membranes (which are trilayer anode/electrolyte/cathode structures) and bipolar plates (which conduct current from cell to cell and provide channels for gas flow into a cubic structure or stack). The bipolar plates are oftentimes made of suitable metallic materials. The cross-flow stack is manifolded externally on four faces for fuel and oxidant gas management.
A radial or co-flow design is another popular design in fuel cell construction. An annular or circular shaped anode and cathode sandwich an electrolyte therebetween. Annular or circular shaped separator plates sandwich the combination of anode, cathode, and electrolyte. A fuel manifold and an oxidant manifold respectively direct fuel and oxidant to a central portion of the stack so that the fuel and oxidant can flow radially outward from the manifolds.
Regardless of the particular fuel cell configuration, the electrochemical reaction between the fuel and oxidant produces electric energy, spent fuel and oxidant exhaust. Quite often, the exhaust gas from a fuel cell is the original reactant gas which has been depleted of the particular migrating species in ionic form as a result of the electrochemical reaction. This conversion of fuel and oxidant to electricity in a fuel cell also produces heat, particularly at high current/power densities, which is removed to maintain the fuel cell at an efficient operating temperature.
Conventional thermal management in a fuel cell forces a cooling medium, either a liquid or gaseous coolant stream, through the fuel cell assembly. Coolants, such as water or air, are used for fuel cell heat exchange depending on the operating temperature of the fuel cell. The coolant enters a fuel cell at a temperature near the operating temperature of the fuel cell. When the coolant passes through the fuel cell, the waste energy from the electrochemical reaction in the fuel cell is carried away by the heat capacity of the coolant. The volume flow of the coolant is closely related to the temperature rise of the cooling medium that is determined by the constraints associated with thermal stress of the components in the fuel cell such as ceramic cells in an SOFC. The heat exchange system typically incorporates mechanical components to facilitate the heat transfer to the cooling medium. Flow channels are routinely employed to keep coolant, fuel and oxidant in their separate passage. In many cases, the cooling medium is also a reactant, e.g., air in an SOFC. The effectiveness of heat exchange is, thus, dependent on the available heat transfer surfaces and efficiency of radiation and convection.
To achieve higher voltages for specific applications, the individual electrochemical cells are connected together in series to form a stack. To achieve higher currents, individual cells are connected in parallel. Electrical connection between cells is achieved by the use of an electrical interconnect between the cathode and anode of adjacent cells. The electrical interconnect oftentimes also provides for passageways which allow oxidant fluid to flow past the cathode and fuel fluid to flow past the anode, while keeping these fluids separated. Also typically included in the stack are ducts or manifolding to conduct the fuel and oxidant into and out of the stack.
In a traditional fuel cell system design, hot exhaust gas from the fuel cell is fed to the heat exchanger, and preheated reactant gas is received from the heat exchanger through insulated piping. For high temperature fuel cells such as SOFCs and molten carbonate fuel cells, costly alloy materials are often used for such piping. The heat exchanger and piping also require considerable installation and maintenance expense, particularly in a multi-stack fuel cell. An effective heat exchange system in a multi-stack fuel cell requires some piping surfaces at each individual stack to maintain an optimal operating temperature for each and overall fuel stack efficiency. One of the problems encountered by a fuel cell design in this aspect can be illustrated by the following example:
When a circulating coolant is used to circulate through the passageways in the fuel assembly, a pump may be required to circulate the coolant. Furthermore, connecting tubes, an expansion tank, radiator, thermal and/or other controls may be necessary to properly complete the heat exchange system. In addition to the added expense and the complexity of integrating the circulating coolant system with the multi-stack fuel assembly, other issues are the need of electricity to operate the pump, the pump is subject to mechanical failure, coolant may become contaminated or ionized resulting in electric short circuits or shunts in the fuel assembly, and the coolant may leak into the fuel cell reaction areas and/or freeze-up. This is a major challenge to a fuel cell thermal designer from the standpoint of weight, cost, structure integrity, maintenance and reliability.
Many have attempted to provide a simpler solution to the heat exchange requirement in a fuel cell assembly. Of particular interest are the following references:
U.S. Pat. No. 3,595,699 focuses on an active cooling method to maintain fuel cell temperature by electrically monitoring the current when it is increased. This allows the control system to react when fuel cell temperatures rise, and by how much before the temperature rises. However, the patent does not cover preheating or thermal integration, and is certainly not related to the overall system efficiency.
U.S. Pat. No. 5,338,622 claims a gas permeable membrane to transfer heat from the fuel cell exhaust to a cooling medium, which is not a reactant of the fuel cell, predominantly by radiation to a separate medium, or by a direct contact with the stack. The use of a gas permeable membrane and a non-reactant heat transfer medium adds complexity to the heat exchange system. A related patent, U.S. Pat. No. 5,462,817, further claims radiative heat transfer from a hot stack (not its exhaust) to a closed loop heat exchanger containing a separate cooling medium that is not a reactant for the fuel cell nor is a product. The cold and hot streams flow in two separate passages (FIG. 5). This decouples flows between the heat exchange fluid or medium and the reactants and products. The oxidant of the fuel is not preheated by the heat exchanger. Energy is needed to circulate the coolants and a separate heating mechanism must be provided to preheat the oxidant.
U.S. Pat. No. 5,340,664 provides for reactant preheat by means of a thermally integrated heat exchange system. The patent claims the use of heat from multiple fuel cell stacks. The heat is exchanged in a double thermal enclosure with incoming air and optionally provides the heat for endothermic reformer which is also enclosed in the thermal enclosure. The heating loops are embedded in the enclosure walls, and heat transfer is mainly by radiation. The heating passages are embedded in the enclosure wall and located between two insulation layers. Similar to U.S. Pat. No. 5,462,817, previously described, the patent requires one hot and one cold flow passage to complete the heat exchange process. The patent further specifies the use of a screw culvert heat exchanger (FIGS. 2 and 3) and plate passages (FIGS. 6 and 7). The claimed system may be thermally efficient, however, the mechanical structures needed for implementation add complexity, cost and weight to the fuel cells.
Other patents simplify the structural requirements for efficient exhaust heat re-uses by mixing fuel cell air and fuel exhaust with fresh colder air and fuel respectively, then recycle back into the cell (U.S. Pat. No. 6,136,462). No heat exchanger is needed for the claimed process. The system mixes heat as well as flow from two separate sources making the flow management quite complex. In yet another simplification attempt, a heat-pipe concept is used for heat exchange where heat transfer takes place through evaporation of an enclosed liquid (U.S. Pat. No. 6,146,779). The presence of a liquid coolant that facilitates heat transfer, again, adds a different complexity to the mechanical structure of a fuel cell.
A different approach to recycle exhaust heat is disclosed in U.S. Pat. No. 6,194,092 by coupling a hydrogen storage tank and a heat exchanger which transfers heat from the stack by convection and radiation in an enclosure. The hot air in the enclosure is forced into the storage tank by the aid of fans. Storage tank, fan and hydrogen are required for the complete system.
A Japanese Patent (JP 60035469), again, uses a separate fluid medium to transfer heat. This medium is not a reactant, as many previously discussed patents. The coolant loop is between some of the cells in a fuel cell stack. Also, the coolant is manifolded to individual cells. The design of such a system is complex and costly.
From the preceding, it becomes apparent that there is a need for a cost effective solution to recuperate waste heat from fuel cell stacks to thermally enhance the performance of fuel cell stacks while improving reliability and maintenance of the fuel cell without the added cost, structure and design complexity. A simple method and apparatus that eliminates the use of separate fluid for heat transfer and employs different heat transfer mechanisms as appropriate is needed to broaden the use of fuel cell stacks in various applications. A method to recuperate the exhaust heat from fuel cell stack without the use of a heat exchanger system or additional fluid is needed. An apparatus, which is flexible to work with single or multiple fuel cell stacks using the same design principle for various fuel cell geometry or arrangement while adaptive to the different structural requirement, is also needed for a fuel cell application.
The present invention provides an apparatus and method to exchange heat from the hot exhaust of fuel cell stacks and use it to preheat the incoming cooler air. The apparatus and method disclosed in the present invention overcomes many of the problems discussed in the prior art.
In one aspect of the present invention, a very simple heat exchanger is used for heat transfer between incoming air, which may be used as a fuel cell reactant, and exhaust heat through a combination of radiation and convection. The apparatus operates in a single passage spiral loop. The heated gases are freely blown over the heating loop. The spiral loop operates outside the fuel cell stack to provide an effective means of heat transfer with structural and design simplicity. Fins and dimples can be further added to the spiral loop to enhance the heat exchange mechanism. The simple design allows the spiral loop to be used in conjunction with a single fuel cell stack or multiple fuel cell stacks.
In another aspect of the present invention, the hot gases may be manifolded and directed out of the fuel cell stack(s) without flowing over the spiral loop. In this case, the heat exchange is by radiation between the hot fuel cell stack(s) and the cold oxidant flowing in the spiral loop.
In another aspect of the present invention, a method of increasing fuel cell reliability and maintainability is disclosed by integrating the recuperating loop into the fuel cell stack(s), simplifying heat exchange by reducing or eliminating the need for external recuperation, surrounding a stack or combination of stacks with a cylindrical spiral tube, directly flowing hot gas over the spiral tube, providing heating to the cathode flow in the tube, and feeding the pre-heated flow to the inlet of the fuel cell stack as a reactant of the fuel cell assembly. In this embodiment, the design complexity associated with a recuperator or heat exchanger at the outlet from the stack is avoided. The requirement for an external cooling medium, which is often a non-reactant fluid, pump, and re-circulation loop for heat exchange, and the maintenance and reliability associated with these components, have been eliminated. Thus, the reliability of the fuel cell assembly may be increased due to the structural and design simplicity.
In another aspect of the present invention, a spiral loop apparatus for the heat exchange at fuel cell stacks is comprised of a tube in rectangular or circular shape, a tube support such as an anchored or creep structure or simple tube supports at the inlet and/or outlet of fuel cell stack(s), and surface augmentation such as dimpling and/or fins if necessary. For high-temperature fuel cells, the tube may be made of any high temperature metal or thermally conductive composites being placed at an appropriate distance from the fuel cell stacks to best facilitate the heat transfer through radiation and convection. The loop structure is an integrated part of the fuel cell structure and the fluid flow in the tube is one of the reactants of the fuel cell electrochemical combustion ingredients.
In yet another aspect of the present invention, a single heat exchange loop is provided as an integral component of the fuel cell stacks. The loop is integrated with the structure of the fuel cell. The spiral loop is used to carry away the fuel cell waste heat which flows in the form of hot gases, with the coolant being a reactant of the fuel cell flowing directly to the inlet of the fuel cell. The loop is designed so that it can work with a fuel stack, either with or without manifolds. In the case of having the loop wrapping around a manifold, the loop is designed to be around the fuel cell stacks, not penetrating or embedded in the fuel cell stacks. The waste heat directly heats the tube wall which, in turn, heats the cooling fluid inside the tube. The heated fluid in the tube feeds into the fuel cell inlet as the preheated oxidant of the fuel cell stacks, eliminating the need of a separate heater structure to preheat the fuel cell reactants and the reliability thereof. Since the tube goes around the stack, not penetrating or embedding in the fuel cell, there is no sealing requirement or structural integration issues in the design aspect of the fuel cell.